The long-term goal of this project is to understand how the brain uses information from the vestibular labyrinth to estimate the spatial orientation and motion of the head. Specifically, these experiments will investigate the on-line interaction between rotational cues derived from the semicircular canals and gravito-inertial cues transduced by the otolith organs. Roll tilt psychophysics and eye movements will be measured in rhesus monkeys while rotational cues are acutely manipulated by electrical stimulation of the posterior canal ampullarynerves. Canal cues will be presented in isolation (with the head in different static orientations and with the eyes at different vergence angles) and will also be coupled to visual (optokinetic) and inertial cues. Tilt perception will be measured with a psychophysical task derived from the subjective visual vertical (SVV), a test which is widely used to evaluate patients with vestibular symptoms. We hypothesize: 1) that rotational cues provided by the semicircular canals contribute to the brain's estimate of head orientation, evidenced by a shift in SVV responses during canal activation that is not explainable by changes in torsional eye position; and 2) that by dissociating the actual and estimated direction of gravity relative to the head, electrical activation of the vertical canals will induce an aberrant estimate of linear acceleration, evidenced by eye movements with the characteristics of a translational vestibulo-ocular reflex. These experiments will help to elucidate how rotational informationderived from the semicircular canals interacts in an immediate, on-line fashion with gravito-inertial cues to produce perceptual estimates of head orientation and reflexivevestibular eye movements. This informationwill contribute to a better understanding of the mechanisms underlyingvestibular symptomatology in patients with labyrinthine and neurologic disorders, includingthe prominentperceptual complaints (vertigo) that dominate the clinical presentation in most patients.